1. Field of the Invention
The present invention relates to a tracking error detector in an optical recording and playback disc apparatus compliant with recording media including, for example, an optical disc and a magneto-optical disc.
2. Description of the Related Art
In an optical recording and playback disc apparatus of the related art, such as a DVD-ROM, as a tracking servo circuit system for irradiating laser light onto a pit sequence with high accuracy, the differential phase detector (hereinafter referred to as a “DPD”) system is known. In this DPD system, when laser light reflected from a disc surface is received by a light-receiving device having divided portions, a phase difference occurs between signals output from the divided portions in accordance with the amount of tracking error, and by feeding back the phase difference information, tracking control is performed.
A tracking error detection circuit (hereinafter referred to as a “DPD circuit”) block of this DPD system serves to read the phase difference information, convert it into an analog amount (for example, a voltage value), and pass it to a digital signal processor (DSP) at a subsequent stage.
The block diagram of the basic configuration of the DPD circuit block is shown in FIG. 1.
As shown in FIG. 1, at a stage prior to the DPD circuit block, there is a light-receiving device 101, such as a photodiode, having four divided portions, which receives laser light reflected from a disc surface, and each portion outputs a data waveform recorded on the disc surface.
The DPD circuit block includes: gain control amplifiers 102a, 102b, 102c, and 102d for amplifying or deamplifying a signal amplitude to an appropriate level when the signal waveforms are very small or large due to variations in the quality of photodiodes and discs; equalizer circuits 103a, 103b, 103, and 103d for accentuating high frequencies of high-frequency signal components contained in a data waveform sequence; high-pass filters 104a, 104b, 104c, and 104d for removing low-frequency components, such as DC offset, from the signal; comparators 105a, 105b, 105c, and 105d for binarizing an analog signal; phase difference detectors 106a and 106b for detecting a phase difference between binarized signals and outputting a signal corresponding to the detected phase difference; an addition circuit 107 for adding together the detection result waveforms; and a low-pass filter 108 for integrating the addition result.
In the example shown in FIG. 1, the signals output from the light-receiving device 101 having four divided portions are in phase between photodiode elements A and C and between photodiode elements B and D. Therefore, there are two systems: one in which a phase difference is detected between the photodiode elements A and B and between the photodiode elements C and D, and these are added together before the low-pass filter 108; and the other in which the signal output from each of the photodiode elements is added in advance like the photodiode device A+the photodiode device D, and the photodiode device B+the photodiode device C, and the phase difference between them is detected.
FIG. 1 shows an example of the former case. The configuration of the circuit for detecting the phase difference between the photodiode elements A and B is basically the same as the configuration of the circuit for detecting the phase difference between the photodiode elements C and D. Therefore, hereinafter, a description is given of the circuit for detecting the phase difference between the photodiode elements A and B. Of course, if the photodiode elements A and B are replaced with the photodiode elements C and D, this also can be applied as a circuit for detecting the phase difference between the photodiode elements C and D.
It is expected that, in the future, as the optical disc reading speed reaches higher multiples and optical discs of the next generation are introduced, the band of a signal that is input will increasingly shift toward a higher-frequency range. In this DPD circuit, in particular, in a DPD circuit formed by a MOS transistor whose speed is slower than a bipolar transistor and whose transconductance Gm is small, the input signal band exceeds the usage band of individual circuits. As a result, the phase characteristics of the circuit become a problem.
Here, how the phase characteristics become a problem is described in detail.
For example, in the comparators 105a and 105b of FIG. 1, that is, in high-gain comparators, it is necessary to output a waveform having a sharp through rate in order to make detection in the phase difference detector 106a at a subsequent stage easier. As stated above, since the transconductance Gm of the MOS transistor is smaller than that of the bipolar transistor, in order to obtain a high gain for realizing a sharp through rate, a configuration in which multi-stage amplifiers are cascade-connected becomes necessary.
FIG. 2 is an illustration showing the operation of a comparator having a configuration in which multi-stage amplifiers are cascade-connected.
As shown in FIG. 2, the input analog signal is gradually clipped at the level of the power-supply voltage while the analog signal passes through these multi-stage amplifiers, and it finally changes to a digital binary signal. In the signal whose amplitude is clipped, distortion occurs, and harmonic components of the original signal frequency components occur. Naturally, in each amplifier, a definite band exists, and there is a demand for an amplifier at a final stage to have a performance capable of enabling the output signal to fully swing to the power-supply voltage level. Consequently, it is necessary to use an inverter circuit for the final stage, etc. Therefore, in general, as the signal passes through the stage number of the multi-stage amplifiers, the region in which the group delay takes a fixed value becomes narrower.
FIG. 3 shows the group delay characteristic in each amplifier output of a comparator having a configuration in which the multi-stage amplifiers are cascade-connected.
If signal components equivalent to or more than those of the band at which the group delay variations start are input, an amount of delay time that differs depending on the frequency components occurs in the output signal. Then, in the amplifier output waveform in which fundamental wave components having a different amount of delay for each frequency and harmonic components that occur due to the distortion are combined, a phase shift that is difficult to predict occurs. The highest range of the input signal when, for example, a DVD is read at 8×reading speed is approximately 35 MHz; and in the 3rd output group delay characteristics of FIG. 3, the range enters a range where the variations have already occurred.
This phase shift poses a significant problem for the DPD circuit block when a large amplitude difference (4 to 5 times as large in the worst case) occurs between two signals for which a phase comparison is to be performed, such as when a variation occurs in the relative positional relationship between an objective lens and a photodiode.
FIG. 4 is an illustration showing a problem when a large amplitude difference occurs between two signals for which a phase comparison is to be performed.
That is, in the comparators 105a and 105b in which amplifiers are formed at many stages, in the example of part (b) of FIG. 4 in which, for example, an input amplitude is comparatively large, when the signal passes through the amplifier at the first stage, the waveform is already clipped at the power-supply voltage, and distortion occurs, thereby generating a phase shift. Then, as the signal passes through the amplifiers at the second stage, the third stage, etc., the phase shift is accumulated. Next, in the example of part (a) of FIG. 4 in which the input amplitude is comparatively small, the waveform is not clipped at the first stage, and neither distortion nor a phase shift occurs. Then, if the waveform is eventually clipped by the amplifier output at the second stage and a phase shift occurs, naturally, the final amount of accumulation of the phase shift becomes smaller than on the large amplitude signal input side in part (b) of FIG. 4.
As a system in which such a DPD system that receives laser light reflected from a disc surface by using a light-receiving device having divided portions and performs tracking control by using the fact that a phase difference occurs between signals output from the divided device in accordance with the amount of tracking error, there is a known system in which characteristics of low-frequency passing means for outputting a tracking error signal in response to a different control operation are switched to improve the accuracy of tracking servo control (refer to, for example, Japanese Unexamined Patent Application Publication No. 10-162381).